The subunit structure of chromatins from a number of types of cells, including chicken erythrocytes, HeLa cells, mouse embryo cells and yeast cells will be examined by nuclease digestion of isolated nuclei and chromatin. Monomeric and multimeric nucleoprotein particles will be isolated and their physical properties and behavior studied. The principal aims are: to understand the substructure of chromatin, in terms of the composition, conformation, and stabilizing interactions with the globular particles; to relate this structure to problems of gene expression and DNA replication. We intend to use mature cells, cells arrested at various stages of the cell cycle, and exponentially growing cells (as in yeast) to determine how chromatin structure changes in the cell cycle.